1. Field of the Invention
The present invention relates to a thermal head and a method of producing the same, and particularly relates to an improvement of a heat-generating resistor layer of the same.
2. Description of the Prior Art
Generally, a thermal head is constituted by a structure of lamination of a heat generating resistor layer for forming dot-like heat-generating bodies, a power feeding conductor layer for feeding the heat-generating resistor layer with electric power, and a protecting film for protecting these layers from oxidization and wear, formed on an electrically insulating substrate such as ceramics with the surface covered with, for example, a thin glass glaze layer.
Conventionally, a thin film of tantalum nitride has been used as the heat generating resistor layer. The tantalum nitride has such advantages that the heat-resisting property is superior, the temperature coefficient is small, and the tightness with the glass glaze layer which becomes foundation is good. In the pattern formation, etching can be easily performed with a mixed acid of nitric acid and hydrofluoric acid, or alternatively, dry etching can be performed by using CF.sub.4 gas or the like.
On the other hand, in the case where the tantalum nitride thin film is used as a heat-generating resistor of the thermal head, the surface of the tantalum nitride thin film is generally covered with an anti-oxdization protecting layer made of a silicon oxide thin film, because the tantalum nitride thin film does not have sufficient anti-oxidization property against thermal oxidization. The tantalum nitride thin film and the silicon oxide thin film are extremely superior in tightness, so that the anti-oxidization property against the thermal oxidization of the tantalum nitride thin film may be increased sufficiently.
Even in the case where the tantalum nitride thin film is used as the heat-generating resistor of the thermal head, however, there have been the following problems.
A curve A in FIG. 1 shows the result of a step-stress test of a thermal head in which the conventional tantalum nitride thin film is used as a heat-generating resistor. The test is a kind of acceleration tests used generally for estimating the thermal stability of the thermal head, in which a suitable pulse voltage was applied to the heat generating resistor for a predetermined time and the change in resistance with respect to an initial value was measured. The applied voltage was gradually raised till the heat-generating resistor was burned out, and the rate of change in resistance in various steps was plotted. The conditions of the step-stress test were such that in the first step the resistance value is measured after a pulse voltage of 20 volts with a pulse width of 1 msec and a repetition pulse period of 20 msec was applied for ten minutes, and in the second step the pulse voltage was raised to 7.5 volts and the resistance value was measured after the pulse voltage was applied under the same condition as the first step. Then, the pulse voltage was successively raised by 0.5 volts every step until the rate of change in resistance .DELTA.R/R exceeds 10%.
An upper horizontal axis in FIG. 1 shows the highest surface temperature at a heat generating portion with respect to various values of the applied voltage. An infrared ray spot thermometer was used for measuring the temperature. As shown by the curve A, in the conventional thermal head using the heat-generating resistor made of the tantalum nitride thin film the resistance value of the heat-generating resistor starts to comes down from a point where the applied power was about 22 W/mm.sup.2 and the heat-generating resistor surface temperature was about 400.degree. C. Now, the surface temperature at the heat-generating portion necessary to generate heat on a thermo-sensitive paper is about 350.degree.-450.degree. C. in an ordinary thermal printer. As the printing speed is made faster, the surface temperature of the heat-generating portion is required to be made higher to exceeds 500.degree. C. In the case where printing is made by using the conventional thermal head using the heat-generating resistor made of the tantalum nitride, the printing density tended to gradually increase as the printing time passed. In some cases, the quality in printing was deteriorated due to printing bleeding, whitely coloring, or the like. These phenomena are caused by the reduction in the resistance value of the tantalum nitride heat-generating resistor at the high temperature as described above. Ordinarily, because the voltage applied to the thermal head is constant while the resistance value of the tantalum nitride heat-generating resistor decreases, the power to be supplied for the heat generation is relatively rapidly increased, so that the surface temperature at the heat-generating portion is raised excessively. Thus, the temperature exceeds the optimum coloring value on a heated paper, so that the quality of printing is deteriorated due to the printing bleeding or the like. Further, there is such a disadvantage that the heat-generating resistor may be burned out because of the undesirably unnecessarily high temperature.